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Rickettsia characteristics include parasitic behaviors but they are not worms or protozoa. Rather, they are bacteria and are therefore prokaryotes. The rickettsia family consists of obligatory intracellular bacteria that are transmitted by arthropod vectors.

In other words, they need to be inside living cells to stay alive.

Dr. Walker explains to listeners

  • How rickettsia diseases interact with the immune system in a variety of ways,
  • Why their lack of motility outside of living cells makes rickettsia treatment challenging, and
  • How the typhus group of rickettsia has affected history and how Dr. Walker hopes to develop a vaccine.

David H. Walker, MD, is a professor in the Department of Pathology and is the Carmage and Martha Walls Distinguished University Chair in Tropical Diseases and Executive Director of the University of Texas MB Center for Biodefense and Emerging Infectious Disease.

He describes various rickettsia causes and symptoms, from the Orientia tsutsugamushi bacteria transmitted by chiggers to others transmitted by lice and ticks.  He explains the means by which scientists determined their inability to live outside of cells and how they are able to observe their invasion and infection with light microscopy.


Their pathogenic effect is widespread, and Dr. Walker describes their effects on World War II and the Vietnam War through the lice vector and typhus spread. Rickettsia symptoms and attacks on the immune system vary greatly and he describes several examples in the two main groups of typhus and spotted fever rickettsia. For example, Rocky Mountain spotted fever bacteria are secreted from the saliva of the tick when it bites.

The bacteria is then taken into the skin through phagocytic cells, spreads to the lymphatic cells, and drains into the blood stream and infects the endothelium cells all over the body.  Dr. Walker has worked with rickettsia for 47 years and his particular focus now is on vaccine research and effective rickettsia treatment.

For more, see his web page at the University of Texas Medical Branch,, and search his name in research systems for publications.

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